1. Field of the Invention
In one aspect this invention relates to electromagnetic forming of metal parts. In a further aspect, this invention relates to electroforming metal parts about a ceramic cylindrical member. In yet a further aspect this invention relates to ceramic and metal parts joined together.
2. Prior Art
In many cases successful applications of ceramic parts to the large scale production runs such as is necessary for automotive and industrial use depend on the use of a cost effective method to join the ceramic part to a metallic part. One example of a metal to ceramic joint which is necessary for utilizing a ceramic part is in a gas turbine or turbocharger structure which requires the attachment of a metallic power shaft to a ceramic rotor. The state of the art can produce a ceramic rotor with a short shaft extending axially from the wheel. However, a metal shaft must be attached to the ceramic portion ot transfer power from the rotor to a drive train.
Coupling the metallic shaft to a ceramic turbine wheel has been at best a difficult and costly task. One method employed in the prior art is a mechanical attachment scheme generally known as a curvic coupling. A curvic coupling is a face spine arrangement with curved radial teeth ground or formed into the face of the parts. In a curvic coupling the coupling memebers can transmit torsional force and are free to absorb relative thermal expansion and axial motion between the two materials. However, such a coupling requires exceedingly close dimensional tolerances and interface contract surface treatment; therefore, the coupling is very costly and difficult to make especially where one member is a ceramic. Such a coupling, while useful as an exercise to prove the concept of a ceramic rotor-metallic shaft assembly, does not meet the requirements of being producible in large volumes and the cost of such structures is prohibitive for use in a large scale consumer product.
Electromagnetic forming is a method which can be used to join a metal to a ceramic. As such, it appears to offer a solution to the problems of the prior art joining methods. Initial attempts to electromagnetically deform a metal shaft about a cylindrical ceramic shaft have shown that the forces generated when the metal shaft is formed rapidly into contact with the ceramic material cause the ceramic to break. Breakage is particularly pronounced where there is a change in the cross-sectional area of the ceramic or areas where the metal has a sharp edge which contacts the ceramic material during forming. Also, it appears that where the metallic sleeve is not uniformly disposed about the periphery of the ceramic material, the metallic sleeve rapidly coming into contact with the ceramic on on one side but not on the other, causes formation of bending stresses on the ceramic. Such stresses frequently cause cracking during the production of the parts.